1. Field of the Invention
This invention relates to vehicle propulsion systems, and more particularly to such systems which are directed to the reduction of polluting emissions and/or improved efficiency in operation. This invention further relates to hybrid systems which combine internal combustion (I.C.) engines and other propulsion means to maximize efficiency in operation and fuel economy while providing minimized emission of pollutants.
2. Description of the Prior Art
As is well known, the conventional spark-ignited, internal combustion engine has dominated personal transportation today for the average American. The engine is inexpensive, is reasonably efficient, and is available with high power in reasonable sizes and weights. The details of its design and its place in the national economy evolved during a period of great availability of inexpensive petroleum-derived fuel. As America's rate of new oil discovery faltered, there seemed to be unlimited supplies of cheap oil flowing from the sands of the Middle East. In the period of generous oil supplies, the main criticism of the internal combustion engine was directed at its emissions of unburned hydrocarbons and oxides of nitrogen that react when exposed to sunlight to make the photochemical smog that Los Angeles had made famous and of carbon monoxide, which is a poison. As the automobile industry has struggled to bring the emissions under control, it has found and advertised, that good fuel economy and low emissions are competing demands, that some of one must be given up in order to get more of the other.
Increased efforts have been directed toward improving the efficiency of various types of power sources by combining different types of prime movers in a single power system. Examples of such combinations may be found in U.S. Pat. Nos. 2,581,596 of Nims, 2,416,942 of Newcomer, and 3,691,760 of Vidal et al; and British Pat. No. 644,759. However, insofar as is known, neither these examples nor any other design efforts have been directed particulary to the problem of combining a steam engine with a piston-type internal combustion engine in a system which effectively improves the overall efficiency by permitting the use of a substantially lower-powered engine with improved fuel economy, and at the same time substantially reducing or eliminating the polluting emissions commonly encountered in the exhaust from the internal combustion engine.
At the present time, I.C. engines are efficient in a range from approximately 1/4 to 3/4 of full load. Even within this limited range, high fuel economy is inconsistent with low levels of emission of pollutants. Low-end torque is not easily available from I.C. engines and is usually provided by complicated and inefficient mechanical transmissions.
The efficiency of the internal combustion engine falls off significantly at loads less than a quarter of full load, and dramatically for loads below a tenth full load. To provide the ten horsepower needed for forty miles an hour cruise from a 200 horsepower engine at high fuel economy is quite inefficient. One could choose to use a smaller I.C. engine, but at the sacrifice of acceleration performance. Most drivers rarely use the full power of their engines, but they really wish to have it available.
Steam engines, on the other hand, provide good low-end torque without complicated mechanical transmissions, and also can be easily used to provide overload power at some sacrifice in efficiency by lengthening the steam admission time during the expansion stroke. In addition, steam engines, because they are external combustion devices, run efficiently at low emission pollutant levels. Steam engines, however, require boilers and condensers of significant capacity and for this reason have been comparatively unsatisfactory for vehicular use.
The difficulties described above with respect to both internal combustion and steam engines are especially disadvantageous in vehicle propulsion systems which require fast start-up and good acceleration characteristics over a broad and continuously varying range of loads, as opposed to a static power plant run at fixed rpm and fairly constant load conditions.